Zimbabweite, a new alkali-lead arsenic tantalate from St Anns mine, Karoi district, Zimbabwe

Zimbabweite (Na,K)₂PbAs₄(Ta,Nb,Ti)₄O₁₈, a new mineral with trivalent arsenic, has been recognized at St Anns mine, southeast of Miami, Karoi district, Zimbabwe, in kaolinized pegmatite and dump material.

The mineral is honey yellow-brown in large crystals, which are as much as 1 x 1 x 2 cm, and is clear pale yellow in thin fragments. Oxidation results in darkening of the color. There is one excellent cleavage, {010}. Zimbabweite has a Mohs hardness of 5 to 5.5, a white streak, adamantine luster, brittle fracture, is not magnetic, and does not fluoresce under either short wave or long wave ultraviolet light. d_m = 6.20(3) and d_c = 6.16 g/cm3. Optically, the mineral is biaxial (+), 2V_Z = about 80°. Mean reflectances in air for an {010} = X-Z cleavage plate are : 589 nm--16.6 %, 470 nm--17.7 %, 546 nm--17.1 %, 650 nm--16.4 %. The indices of refraction determined by immersion methods are all greater than 2.10. Optic axis dispersion is very strong with ν > r, a = Z, b = Y, and c = X. The mineral is very pale yelow and is moderately pleochroic and X = pale yellow brown, Y = light reddish brown, and Z =reddish brown.

Zimbabweite is insoluble in common acids or bases. A chemical analysis yielded, in weight percent, Ta₂O₅ 46.5, As₂O₃ 26.5, PbO 15.0, Nb₂O₅ 4.8, Na₂O 3.1, K₂O 1.5, TiO₂ 1.4, BaO 0.4, UO₂ 0.3, Bi₂O₃ 0.2, H₂O (total) 0.19, SnO₂ 0.1, F 0.04, SrO 0.02, total 100.05. A calculated formula is :
(Na_1.51K_0.48Ba_0.04)_Σ2.03Pb_1.01(As_4.03Bi_0.01)_Σ4.04(Ta_3.17Nb_0.55Ti_0.26U_0.02Sn_0.01)_Σ4.01O₁₈.

Zimbabweite is orthorhombic, space group is Ccma or Cc2a, with a = 12.233(2)Å, b = 15.292(2)Å, c = 8.665(2)Å, V = 1621. 0(4) Å₃, with Z = 4. No structural relationship between zimbabweite and any other tantalate minerals is apparent.